Light control signal generator



July 26, 1960 J. B. POWERS LIGHT CONTROL SIGNAL GENERATOR 3 Sheets-Sheet 1 Filed Oct. 29, 1957 INVENTOR. JOHN E P0 1525 ark July 26, 1960 Filed Oct. 29, 1957 J. B. POWERS LIGHT CONTROL SIGNAL GENERATOR 3 Sheets-Sheet Z Ournur 2 7 a! "To z j u 0urpur 1/ 57 li n m' i6 2! "tar A! M 007007 I :1 1 INVENTOR.

JOHN 5. ea/:95

July 26, 1960 J. B. POWERS 2,946,394

LIGHT CONTROL SIGNAL GENERATOR Filed Oct. 29, 1957 3 Sheets-Sheet 3 1N VENTOR. 1 ./amv 5. Poe/:25

o uzww q' firraeA/ifi LIGHT CONTROL SIGNAL GENERATOR John B. Powers, Los Angeles, Calif., assignor to The Regents of The University of California, Berkeley, Calif., a corporation of California Filed Oct. 29, 1957, Ser. No. 693,137

11 Claims. (Cl. 250-210) This invention relates to apparatus particularly adapted for selecting between components according to their color so that objects of diflerent colors or shades may be distinguished one from the other in color sorting.

Particularly in the sorting of fruit, of which lemons are an illustration of a form with which this invention can be used, it is important that the individual fruit pieces be segregated or sorted accurately in accordance with their varying degrees of maturity. The lemons when simultaneously picked, even from a single tree, are usually found to be of various degrees of maturity. Some lemons are of the variety which is known to be far from maturity, such as the dark green lemons; others are lighter green in color and closer to a mature state; others are of the socalled silver variety and quite close to a mature state; while still others are of the yellow variety and truly both ripe and mature. Consequently, while all'of these lemons are in harvestable condition, the different degrees of maturity make it desirable to sort and segregate the lemons according to surface coloration, which is the present indication of the actual maturity of the fruit and of the time they can be preserved before marketing.

In accordance with teachings of this applicant, one particularly efiective sorting operation by which even closely similar surface color shades may be distinguished has been provided by developing a plurality of signal pulses from which a ratio of the light reflectance of the surface of the object to different selected ilurninating wavelengths spaced in the spectrum may be established. To achieve this effect light sensitive translating devices or phototubes are illuminated by light reflected from the objects to besorted.

Output signals of one form are obtained when the objects 'to be classified are first illuminated individually by light confined to one narrow band located toward one end of the spectrum. Other output signals are developed from the light sensitive translating devices when they are subjected to illumination by light reflected from the same objects when illuminated by light occupying a second narrow band within the spectrum, but which second hand appears in a different portion within the spectrum from the first illumination. The light wavelengths to illuminate the objects is then shifted in any appropriate way so that as each object passes an exploring point it is illuminated by each selected light wavelength in sequence. The illuminating light changes from instant to instant by the :control of this invention so that the objects are alternately illuminated by substantially monochrome light of different spectral composition. The illuminating light :under such conditions is then shifted at any selected rate between the two illuminating substantially monochromatic light compositions with at least' one exposure or illumination of the object to be sortedat the exploration point with each illuminating light composition. The result is that reflected light from the subject produces output signal energy from the light sensitivedevices into which the reflected light falls with at least one output signal occurring during object illumination at each substantially monchrornatic light composition. In this way the reflect- 2,946,894 Patented July 26, 19.611

ance of the objects to light at eachrilluminating wavelength is determined.

The signals serving to control the sorting operation are termed by this applicant to be signals indicative of the index of variation of reflectance, which is also and frequently called the IVR. Broadly, the index of variation of reflectance is equal to the ratio provided by the difierence in signal output from light-sensitive elements illuminated by light reflected from the object when the object is subjected to illumination at two distinct wavelengths of light compared to a signal value which is represented by a constant times the. maximum signal, plus the quantity 1 minus the selected constant times the minimum signal, where the selected constant can be chosen or selected as any value in the range between and including each of zero and unity. Mathematically this may be expressed, illustratively, if the maximum signal is assumed to be R and the minimum signal is assumed to be R in a form where the index of variation of reflectance (IVR) is represented as It now K is assumed to equal zero, then I VR R2 I Similarly, if K is assumed to equal unity (1), then Other values of K will provide an index of variation of reflectance (IVR) which can readily and similarly be computed.

In the operation of color sorting devices such as those used to sort lemons wherein the segregation of the components to be sorted is established in accordance with measurements of the index of variation of reflectance, a

multiplicity of photoelectric tubes is so located that as each object to be sorted comes in sequence within the exploration region it is practically surrounded by a group of light-sensitive elements or photoelectric tubes so that the photoelectric tubes receive light reflected from all parts of the object, that is, top, bottom, all sides, as well as at various angles. The precise number of photoelectric tubes may vary but there should be a suflicient numher that the combined output represents generally the integrated light reflectance over the complete object surface. Then when the object is illuminated by light of difierent wavelengths (i.e., substantially monochromatic light of diflerent spectral composition) this light is reflected from the object in substantially all directions so that the reflected light is quite independent of the object size, shape, 7 orientation, surface characteristics, or the like. At the same time, since the light is reflected to a multiplicity of photoelectric tubes, the reflected light is measured by the combined output of these phototube's, all or groups of which are connected with their outputs in parallel. For determination of the index of variation of reflectance, each of the signals produced as a result of illumination of the object at a selected light wavelength is subdivided or chopped at'a frequency much higher than the rate of shift between the two illuminating wavelengths. This etlectively provides distinct signal pulses of amplitude proportional to the light reflectance of the object at the encase particular light wavelength and yet it provides ways by which the desired carrier frequency or chopping frequency can be recovered for the purpose of utilization in the controlling circuit instrumentalities.

In the preferred form of operation, according to this invention, the phototube current produced as a result of object illumination at a selected light wavelength is interrupted or chopped to produce a selected carrier frequency. In order to avoid the simultaneous generation of a spurious and undesired background signal which might occur because of the unavoidable internal capacitance, of the phototubes, the phototubes are connected in a bridge circuit. Usually a plurality of paral'lelly. connected phototubes form one arm. o a, ridge and other phototubes, also parallelly connected, formadjacent arm of the bridge. The third and fourth arms of v the bridge are formedby suitable impedance elements and appropriate operating bias is supplied to the phototubes along the diagonal between he junction of the, two bridge arms formed by each oj'the sets of parallelly connected phototubes and the junction of the two impedance elements which form the third and fourth arms of the bridge. Exciting voltage at one-half of the desired carrier frequency is applied by way of coupling to the impedance elements so that it appears across the opposite diagonal of 'the bridge whereby, for conditions of bridge balance and in the absence of phototube illumination, no output is developed along the diagonal from which the biasing voltage is applied. At times when light strikes the phototubes current flow is produced therethrough, and current; pulses at the frequency of the exciting voltage flow through the bridge arms and thence through the diagonal along which the biasing voltage is applied. However, since the exciting voltage is out-of-phase at opposite ends of the diagonal over which it is applied, there appear in the diagonal of the bridge from which biasing voltage is applied output pulses at double the frequency of the exciting voltage, which pulses are of an, amplitude proportional to the reflectance of light from the object at the instantaneous light wavelength at which the object is being illuminated to reflect light therefrom to the phototubes. Any voltage appearing across the bridge diagonal from which biasing voltage is applied by reason of slight bridgeunbalance occurs at a frequency. an; octave removed from the repetition rate of the output current pulses and is easily removed by tuning techniques to be described later.

The d v c d. e b i g ir t. a in y a en.- erally as a; full wave rectifier whoseoutput is; controlled y igh ching the pho ube Pre i e b la e f the bridge is not necessary and the effect of the capacity of the phototubes in the two bridge legs is generally neutralized. Appropriate adjustment of thephototube bias and the magnitude of the exciting voltage generally serves to produce voltage saturation in the phototubes during a large fraction of their total conducting periods and to interrupt the flow of current during similar time intervals, thus providing a close approach to a squarewave output.

From the foregoing, one of the objects of the invention. is to provide an improved light modulated, circuit wherein a carrier signal is produced whose amplitude is ameasure of light intensity.

Another object of the invention is to provide im" proved circuitry for light modulated signal generators used in color sorting operations.

A further object of the invention is to. nullify the capacitive eifects of phototubes in color sorting opera: tions where the phototube: outputs represent the signals from which control voltages or ratios. are, derived.

Other objects and advantages of the invention will become apparent from a consideration of the accompanying drawings in which Eig;.; 1 represents schematically one circuit for, trans,-

wherein the bridge circuit is. capacitively balanced;

Fig. 3 is a circuit diagram, of a form of the invention 7 wherein the light modulated circuit comprises a bridge circuit inwhich parallel ly connected phototubes comprise at least two arms of the bridge and wherein negative bias is applied to the phototube anodes.

Fig. 4 is a circuit somewhat similar to the showing of 'Fig. 3 where the phototubes, when illuminated, serve to determine the amplitude of the output Signal and wherein positive bias is supplied to the phototube anode elements;

Fig. 5 is a circuit diagram similar to Fig. 4 but Wherea ms output is derived across a difierent form or" tuned circuit; and

Figs. 6 and 7 are series of curves to explain the operation of the circuits of Figs. 3, and 4 respectively.

Referring now to the drawings for a further understanding of the invention and first to the circuit of Fig. 1, exciting voltage at a selected frequency is applied at a pair of; input terminals 11 and fed through a coupling transformer 13 to a multiplicity of phototubes 15, 16, 17, etc., and thence to a tuned output circuit 27- comprising the inductance element 29 and its shunting capacity 31. The output circuit 27 is tuned. to resonance at the frequency f and the voltage developed to f flows through the resonant circuit 27 and produces at the output terminals 25 a carrier voltage at the frequency f whose amplitude is a measure of the total photoemitted current. The phototubes generally operate as half-wave, light-modulated rectifiers.

The phototubes 15, 16, 17, etc., are placed relative to thevobjects to be explored in such relationship that reflected light from all regions of the object (that is, its top, bot-tom, all sides and various angles) reaches the phototubes which then are illuminated by light reflected from all regions of the illuminated object with the reflected light controlling the current flow therethrough. The resultant current at the output is determined by the average. or integrated output of all of the phototubes and thus can represent the reflected light over substantially the whole object surface.

As the combination has been shown by Fig. 1, the

capacity. of the group of phototubes is schematically representedin dotted outline by the capacity element 20. Due to the fact that the input frequency f may have a relatively high value,,for instance something of the order of 65 kc., as an illustration, the phototube capacity is often suflicient, irrespective of whether or not the phototubes are illuminated, to provide a transfer path between the input transformer 13 and the output terminals 25. Thus a continuous background voltage at the frequency f and; independent of phototube illumination is produced across the output terminals 25 which may exceed by thousands, or even hundreds-of-thousands of times the output voltage producedat the terminals 25 by photoemissive action. Under conditions of low illumination of the phototubes this background voltage effectively masks the light-modulated output signal and the device generally fails to perform as a. light-modulated signal generator.

One way in which the signal masking eflect .of the capacity-coupledbackground voltage can besubstantially reduced is shown in the circuit modification of Fig. 2. Here the phototubes 1 5, 16, 17, etc. form one leg of a bridge circuit, an adjacent leg of which is formed by the capacity element 41 suficient in magnitude to balance the phototube capacity for the non-illuminated condition thereof. The two other legs of the bridge circuitare formed from an impedance element 43 which is tapped at its center point. One of the outer terminals of the impedance element 43 is connected to the phototubes and the other terminal is connected to one terminal of the balancing capacity 41.

With the arrangement of Fig. 2 an output signal voltage is generated across the circuit 27 made resonant to the frequency f by the action thereon of current pulses pro duced by the combined action of light reflected from the object and exciting voltage from the transformer 13 on the phototubes in the same manner as that described above for the circuit of Fig. 1. This output signal voltage of frequency f appears at output terminals 25. However, in this case current from the input transformer 13 reaching the resonant circuit 27 through-the internal capacity of the phototubes is substantially cancelled by current of opposite polarity coupled from the-transformer to the resonant circuit through the balancing capacity 41. This is due to the fact that the potentials at the outer ends of the inductive element 43, of which one connects to the phototubes and the other to the capacity 41, are of opposite polarity.

Clearly the degree to which the bridge arrangement of the circuit of Fig. 2 is effective in suppressing the capacity coupled background voltage at the output terminals 25 is dependent on the preciseness of balance of the bridge. In practice, use of the circuit of Fig. 2 in place of the circuit of Fig. 1 can result in a reduction of background voltage by a factor of 1000 or even 10,000. Even with a reduction of this magnitude it is found that the minimum practical residual background voltage attainable with the circuit of Fig. 2 is still of such magnitude as to effectively mask the output signal due to photoemission under conditions of low illumination ofthe phototubes.

By the form of the invention shown in Fig. 3 the effect of phototube capacity is balanced out'by other phototubes and wherein any residual capacity-coupled background voltage appearing at the output terminals 25 occurs at a frequency an octave-removed from the frequency of the desired signalproduced by the action of light on the phototubes. .In this case the desired signal is readily separated from the capacity coupled background voltage by the tuned output circuit. The circuit which is schematically represented is so arranged that one leg of the bridge comprises a plurality of parallelly connected phototubes 44 and 46, of which only two are shown for convenience of illustration; The adjacent leg of the bridge comprises additionalparallelly connected phototubes 48 and 50, of which also only two are shown for illustration convenience. The phototubes having substantially the same capacity serve as a capacity balance on the bridge. In this instance the input signal frequency f is also supplied at the input terminalsll and fed through the input transformer or coupling 13 to the secondary formed by the impedance element 43, of which equal portions connectbetween. a midpoint 52 and the phototubes 44 and 46 on the one end and phototubes 48 and 50 on the other end. Bias to the phototubes is supplied by a source of direct current 33 connected between ground 35 and the midpoint 52 of inductance element 43; The input signal frequency is thus supplied across the diagonal of the bridge between the connection of the outer end 54 of the inductive element 43 and the phototubes 44'and 46 and the connection of the second outer end 54 of the inductance 43 and the phototubes 48 and 50.

Output voltage is derived across the diagonal of the bridge between ground 35 (effective as if at the point 52) and the junction point 56 of the parallelly connected phototubes 44 and 46 and the second group of parallelly capacity elements 55. The'circuit 51 in this instance is tuned to twice the input frequency or to a frequency value of 2 Voltage developed across this tuned circuit between points 56 and 35 appears at the output terminals 25; 'In this form of showing the phototubes have their anode elements 39 biased negatively by the direct current source 33. The circuitry of Fig. 4 is generally similar to that disclosed by Fig. 3 except that in this instance the anode elements 39 of the phototubes are biased posi-' tively by the source 33, as indicated on each of the figures by the plus and minus signs.

Considering now the diagrammatic showings of Figs.

6 and 7 it will be observed that the curves of Fig. 6 are 'related to the operation of the bridge c-ircuit disclosed by Fig. 3 while the curves of Fig. 7 are related to the bridge circuit operation as described by Fig. 4. In each instance it will be noted that the input frequency f is applied through the coupling transformer in a way such that voltage E at the end-54 of the inductive element 43 is 180 out-of-phase with respect to the voltage E available at the outer point 54' of the inductive element 43. The amplitude of the impressed voltage wave occurring at the frequency f is represented on Fig. 6 as being of a value-represented between the crest of the wave E measured to the dot-dash line E The zero potential is represented by the line marked zero and the biasing value efiective in the negative sense with respect to the phototubes 44, 46, 48 and is represented on Fig. 6 by the difference between zero and E so that the impressed voltage wave oscillates about a steady state value represented by the dot-dash E In this figure and the upper portion thereof, designated by the curve (a), the dash line E represents the voltage required to establish a condition of voltage saturation within the phototubes. E is the value of phototube polarizing voltage above which the phototube current is a function of illumination provided for the illumination. It will be noted from the consideration of portion (a) of Fig. 6 that the voltage wave E exceeds the saturation value E' of the phototubes in'the portion of the wave which crosses the line E The current through the phototubes 44 and 46 designated by I and measured from zero value is shown by the curve (b) and it will be seen that for a substantial portion of each cycle of the impressed input frequency wave occurring at the frequency f no output results due to the impressed voltage E Similarly, for the phototubes 48 and 50 which receive the impressed voltage E shown by the dash line (a) in Fig. the current output, designated as I is represented by a dash line, on the curves of Fig. 6 designated (0). Thecurrent pulses I like the current pulses l occur at the frequency f of the impressed input and since these currents add in the tuned circuit 51 in the bridge diagonal across which the output terminals 25 are connected, it will be observed from the curve (d) of Fig. 6 that the total output for the illuminated condition of the phototubes 44, 46, 48 and 50, etc. is represented by the curve I +I and that the pulses of current occur at twice the frequency of the impressed wave f From the curves (b) and (c) and (d) of Fig. 6 it will be observed that a portion of each curve is flattened out at the top which indicates that the phototubes have reached a saturation state, as indicated. The curve (d) of the group represents the summation of the curves I and 1 designated on the figure'as curves (b) and (c) so that the pulses I +I occur at twice the frequency of either of thepulses developed under'the control of the applied voltage waves E and E This represents one state of illumination of the phototubes and it will be under- 1. stood that other light, activation will provide different amplitude outputs.

With the: circuitarrangementas; provided by the poling ofthejsource 3 3,; as; in Fig.;4, thetypeof operation exemplifiedbythe curves of Fig.;,7 apply. From Fig; 7 it will be appreciated that thevoltage waves E and E representtheinput:frequency-wave h, as instantly available at the ends- 54 and 5-4 of the impedance element 43,

for instance. These curves, as are the curves. of Fig. ,6, are drawn to show that'the, alternating current components of the input voltages E and B are in, opposite time phase; in contrast to that of Fig. 3 that the biasingvoltagerepresented, by the level E shown as the dash-dot line on:

curve (a) is positive so that saturationoccurs within the phototubes 44, 46, 48 and,50 at a level shown as E plotted with respect-to the zero line, Under the, circumstances output-current from the phototubes will flow at all portions of'thewave, except that represented by the period of time when the activating wave at the frequency f dipsbelow the line of zero potential which can be seen from the curves (b) and (0) so that the output currents I and I rrespectively are generated when voltage waves E and B are applied andthephototubes are illuminated. The combined output-from the two waveforms, is designated also in Fig.7 by the curve (d). As in the typeof operation depicted by the curves of Fig. 6, the total output current consists of a series of unidirectional pulses occurring atarepetition rate equal to 2f It will be observed thatthere is a slight advantage to be offered from the form of operation where thepositive polarity voltage is applied to, the phototube anodes. because the waveform of the current pulses in curve, (d) of Fig. 7 produces higher impulse values than those of curve (d) of'Fig. 6 and thus produces a somewhat higher output signal voltage for a, given exciting. voltage and phototube illumination. The type of operation, is, however, generally optional and for some purposes one form.

offersadvantages ascompared to the other..

In the refinement of the invention exemplified by the circuit of Fig.5 the, voltage source 33 ispoled with re-' spect to the phototubes in a manner similar to that shown by Fig. 4. However, additional advantage is gained from the arrangement of Fig. 5 in that the resonant load circuit has been modified to, provide an extremely low impedance path to groundfor any capacity-coupled currents at the frequency f which flow through the load circuit by reason of slight unbalance of'the bridge circuit. The arrangements of circuits shown by Figs. 3. and: 4 provide that the tuned output circuit, 51 comprising the inductance elements 53 and the capacity-element 55' is tuned to a.

frequency 2; which is twice the input, frequency, resulting' of course from the fact that the output pulses as shown by portions. (d) ofeach of Figs. 6 and 7 occur at double the frequency of either the, pulses 1,, as per curve (1)) or the pulses I as per curve (0). In the circuit of Fig. 5 the output is derived across the bridge diagonal between similar connections but is derived across a connection of a plurality of capacitive and inductive elements connected in such a way that one inductive and capacitive path is tuned to series resonance at the input frequency f and the combination as a whole is tuned to parallel resonance at the double frequency 2 In this way most of the undesired frequencies are eliminated from the output available at the output terminal points 25 Considering specifically the circuitry of'Figpf the output pulses are derived across the diagonal between the point5'6 and ground 35,- as in Figs. 3 and 4. The combination of inductance and capacitance elements forming the network 57 consists of the inductor 59 and the series connected capacitor 61 connected between point 56 and ground 35. These; elements tune in series resonant fashion to the inputfrequency f Then, by-meansof the combined-effect of the series; resonant path 59, 6.1 and its. shunting condenser or; capacitor; 63'; and; shunting; induc-.v

The circuitarrangement of Fig, 4 provides,

-. nceted to point- 56.

g. tor 65, the combination as a whole is made parallel resonant to the double frequency 2f 'As thearrange. mentis shown, of course, the .dc path is provided at all, times through the inductive element '65. However, it

- will be appreciated that various combinations of induc tance and capacitancemay be used" to effect the desired end result. 7

It will be clear from the foregoing descriptions that the circuits of Figs. 3, 4 and 5 will be operative with theconnections to each bank of phototubes interchanged. Thus, in any of these circuits the ends 54 and 54 ofthe impedance element 42 can be connected to the cathodes of phototubes 44 and 46 and the phototubes 48 and 50 respectivelyand the anodes of all phototubes may be con- With this connection and with suitable adjustment of the biasing voltage from the source 33, the circuit of Fig, Swill perform in the manner described aboveforthe circuits of Figs.'4 and 5,, and they circuits ofjFigs; 4; and 5 will perform in the manner dee scribed for the circuit of Fig. 3. Furthermore, the phototube biasing voltage may be obtained in any convenientway such as, for instance, connecting the biasing source; 33 between the resonant circuit 51 and the ground point- 35 instead of inthe position shown, without in any way affecting the operation of the circuits.

It will be further recognized that the circuits of Figs. 3, 4 and 5 will perform generally as described above to produce output signals at twice the frequency of the applied exciting voltage even when the biasing voltage is completely removed such as by short-circuiting' the biasing source 33. In this case, the duration of individual pulses of the currents I and I in the two photosensitive arms, of the bridge willbe equal to onehalf the period ofthe exciting voltage as in the circuitsof Figs. 1 and 2' but, in contradistinction to the circuits of Figs. 1 and 2, the pulse repetition rate of the total output current will'be equal to 2h. The use of a biasingvoltage as indicated in Figs. 3, 4 and 5 is usually prefer: able to operation without bias since the duration and shape of the output current pulses can thereby be con? trolled to maximize the component of the output current occurring at the frequency 2h. Occasionally, however,

itis desirable to eliminate the biasingsource for purposes of circuit simplification.

Having now described the invention, what is claimed? and is desired to be secured by Letters Patent is the following:

1. A light-controlled signal generator comprising a bridge circuit including a pair of photoemissive cellsadapted for illumination by a single light source andconnected in seriesopposition to form two adjacent bridge" arms and a pair of substantially equal impedance elements connected in series therewith to form a closed circuit, means to apply an oscillating voltage ofifrequency across the bridge diagonal spanning the connected impedanceelements, and means to derive at the otherbridge diagonal a signal of frequency 2 controlled in amplitude by light impinging on the photosensitive surfaces.

2. The device claimed in claim 1 including, in addition, means for applying on both photoemissive cells a com.- mon biasing voltage lower in magnitude than the amplitude of the applied oscillating voltage and of a polarity" with;

lightesensitivemejans, connectedtheretoto form the other two adjacent bridge arm's,-each light-sensitive'means being adapted tob'e' modulated'byvlight from a common light source, means for applying a common biasing voltage'upon the separate light-sensitive means, ,means to apply an oscillating voltage of frequency f and of an amplitude'greater than the bias voltage to each bridge arm including the impedanceelements with the polarity of the applied voltage being instantaneously ofopposit'e phase at the junction of eachimpedance elementand the light-sensitive means of the adjacent bridge arm, and means to derive at the bridge diagonal between the junction of the lightsensitive means forming adjacent arms of the bridge and the junction of the impedance elements forming the bridge arms a signal of a frequency of 2 modulated in amplitude by light impinging upon the light-sensitive means to determine the current flow therethrough.

5. A light controlled signal generator comprising a bridge circuit including a pair of impedance elements series connected to form two adjacent bridge arms and a plurality of parallelly connected light-sensitive means connected to form each of the other two adjacent bridge arms, each light-sensitive means being adapted tobe modulated by light from a common light source, means for applying a common biasing voltage upon all of the light-sensitive means, means to apply an oscillating volttage of frequency f and of an amplitude greater than the bias voltage to each bridge arm including the impedance elements with the polarity of the applied voltage being instantaneously of opposite phase at the junction of each impedance element and the light-sensitive means of the'adjacent bridge arm, and means to derive'at the bridge diagonal between the junction of the light-sensitive means forming adjacent arms of the bridge and the junction of the impedance elements forming the bridge arms a signal of a frequency 2 modulated in amplitude by light impinging upon the light-sensitive meanstto determine the current flow therethrough.

6. A light controlled signal generator comprising a bridge circuit including a pair of series connected impedance elements forming two adjacent arms and a plurality of unilaterally conducting light-sensitive means connected to form the other two adjacent arms, said light-sensitive means each having a photosensitive cathode and an anode, means for applying a common biasing voltage upon all of the light-sensitive means with the bias of a polarity to promote current flow with illumination, means to apply to the impedance elements an oscil l-ating voltage of frequency f and of an amplitude greater than that of the bias to cause a periodic interruption of current flow through the light-sensitive means with illumination thereon, and means to derive at the bridge diagonal rality of unilaterally conducting light-sensitive means connected to form the other two adjacent arms, said light-sensitive means each having a photosensitive cathode and an anode, means for applying a common biasing voltage upon all of the light-sensitive means, with the bias of a polarity to inhibit current flow with illumination, means to apply to the impedance elements an oscillating voltage of frequency f and of an amplitude greater than that of the biasing voltage to permit current flow through the light-sensitive means during portions of the cycle when the bias voltage is overcome and during illumination of the light-sensitive means and thereby to develop across the bridge diagonal between the junction of the impedance elements a resultant voltage of a upfon'the light-sensitive means to determinethe current flow therethrough." v q 8. ,A light-controlled signal generator comprising a bridge circuit including a pair of photoemissive cells adapted for illumination by a single light source and connected in series opposition to form two adjacent bridge arms and a pair of substantially equal impedance elements connected in series therewith to form a closed circuit, means to apply an oscillating voltage of frequency 7 across the bridge diagonal spanning the connected impedance elements, means to derive at theother bridge diagonal a signal of frequency 2 controlled in amplitude by light impinging on the photosensitive surfaces, and an oscillatory circuit parallel resonant at the frequency 21 connected in the bridge diagonal extending between the junction of the connected photoemissive cells and the junction of the connected impedance elements.

9. A light-controlled signal generator comprising a bridge circuit including a pair of photoemissive cells adapted for illumination by a single light source and connected in series opposition to form two adjacent bridge arms and a pair of substantially equal impedance elements connected in series therewith to form a closed circuit, means to apply an oscillating voltage of frequency f across the bridge diagonal spanning the connected impedance elements, means to derive at the other bridge diagonal a signal of frequency 2 controlled in amplitude by light impinging on the photosensitive surfaces, and a plurality of capacitive and inductive elements connected in the bridge diagonal between the junction of the connected photoemissive cells and the junction of the connected impedance elements, the said combination of capacitive and inductive elements being series resonant at the frequency f and being parallel resonant at the frequency 2].

10. A light controlled signal generator comprising a bridge circuit including a pair of impedance elements series connected to form two adjacent bridge arms and light-sensitive means connected thereto to form the other two adjacent bridge arms, each light-sensitive means being adapted to be modulated by light from a common light source, means for applying a common biasing voltage upon the separate light-sensitive means, means to apply an oscillating voltage of frequency f and of an amplitude greater than the bias voltage to each bridge arm including the impedance elements with the polarity of the applied voltage being instantaneously of opposite phase at the junction of each impedance element and the light-sensitive means of the adjacent bridge arm, means. to derive at the bridge diagonal between the junction of the light-sensitivevmeans forming adjacent arms of the bridge and the junction of the impedance elements forming the bridge arms a signal of a frequency of 2] modulated in amplitude by light impinging upon the lightsensitive means to determine the current flow therethrough, and an oscillatory circuit parallel resonant to the frequency 2 connected in the bridge diagonal between the junction of the connected light-sensitive means and the junction of the connected impedance elements.

11; A light controlled signal generator comprising a bridge circuit including a pair of impedance elements series connected to form two adjacent bridge arms and light-sensitive means connected thereto to form the other two adjacent bridge arms, each light-sensitive means being adapted to be modulated by light from a common light source, means for applying a common biasing voltage upon the separate light-sensitive means, means to apply an oscillating voltage of frequency f'and of an amplitude greater than the bias voltage to each bridge arm including the impedance elements with the polarity of the applied voltage being instantaneously of opposite phase at the junction of each impedance element and the light-sensitive means of the adjacent bridge arm, means to derive at the bridge diagonal between the junction of lated in amplitude by light impinging upon the lightsensitive means to determine the current flow therethrough, and a plurality of capacity and inductive elements connected in the bridge diagonal between the junction of the photosensitive elements and the junction of the impedance elements, the said combination of capacity and inductance elements being series resonant to the frequency f and being parallel resonant to the frequency 2f.

UNITED STATES PATENTS Barber Oct. 29, 1940 Hurley July 27, 1948 Herbold Nov. 22, 1949 Nyman July 17, 1951 Schroeder et a1. Apr. 22, 1952 Lehovec July 7, 1959 Beck Oct. 6, 1959 

